Resembling, characteristic of, or relating to a coronet; having the quality or style of a coronet.
Coronet plus the adjective suffix -ty (or -y), which transforms nouns into adjectives describing their quality or resemblance.
Adding -y to make adjectives is one of English's favorite tricks—you can take almost any noun and slap -y on it to describe its quality, which is why 'coronetty' makes sense even if it's rare!
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